While some sources agree with you: https://www.thefreedictionary.com/mediums
Both Media and Mediums are plural forms in common usage. Thus: https://www.merriam-webster.com/thesaurus/mediums has mediums variants: or media plural of medium
ex: https://www.uspto.gov/kids/MiddleSchool-HowWellSoundTravels....
And really, English just evolves over time so these kind of rules don’t actually mean anything.